I'm not sure there is a "correct ratio" for a diesel or how you'd measure it as the air intake isn't throttled, unlike a petrol. As Shayne has said, running lean means less power but also cooler running. EGT's are the important figures on a diesel, ideally you want an EGT guage so you can tweak the fuel to keep the temperture down during break in, especially if you're uprating the turbo for more boost.